In motor vehicle engineering, many devices, in particular valves such as brake control valves, anti-lock brake valves, trailer brake valves and the like, are activated by electrical pulses and, in particular, pulse-width-modulated signals (referred to below as PWM signals). The control devices which are used for this purpose usually contain a program-controlled microprocessor which activates the corresponding valves via an output stage (driver circuit).
In safety-critical applications, such as in brake valves of motor vehicles, it is necessary to detect as quickly as possible faults which occur, such as failure of the driver circuit, line breaks, short circuits, grounding faults and similar. For this purpose, costly checking circuits are known which monitor and signal possible faults.
For example DE 42 15 075 C2 discusses a method for activating an electrical component with at least one microcomputer with two control inputs. Signals are transmitted unidirectionally on a first control line and bidirectionally on the second control line. The second control line is also used here as fault feedback line, and the bidirectional connection is capable of differentiating two different LOW levels, with corresponding levels being compared using a comparator and conclusions about a fault state being drawn therefrom.
Further monitoring circuits are discussed in German patent documents DE 197 58 101 C2, DE 42 02 601 A1, and DE 29 38 344 A1.